The present invention relates to the production of low density particulate fillers suited for introduction into polymeric materials to lower the bulk density of the mixture.
Polymeric resins that are widely used in commercial products normally have relatively high densities. When such materials are used in applications where reductions of the bulk density would not detrimentally affect the mechanical properties of articles made from the resins, the density of the material is reduced. One means of reducing the density of such materials is to mix low density particles with the resin. The present invention is applicable to this technique for density reduction.
Expanded perlite is a particulate, low density material that is available in commercial quantities and at commercially viable prices. The expanded form of perlite is produced by the heat treatment of perlite ore. Perlite ore is an igneous mineral consisting primarily of silica and small amounts of alumina in a structure combined with a small percentage of water. When perlite ore is heated, generally to a temperature exceeding 1600.degree. F., the water is abruptly vaporized and released from the mineral structure. Proper selection of the ore size, heating rate and temperature of the heat treatment will result in a final expanded product having an apparent density of approximately 6 to 15 lbs/cu ft as compared with the density of the perlite ore, about 145 to 150 lbs/cu ft. Since a typical density of a polymeric resin is about 75 lbs/cu ft, the addition of expanded particulate perlite will have a markedly favorable effect on the bulk density of a mixture of resin and particulate expanded perlite.
In practice it is difficult to achieve the reductions in density theoretically possible with additions of particulate expanded perlite because of the relatively poor mechanical properties of the expanded perlite. The process of mixing the expanded perlite particles with the plastic resin normally results in the fracture and attrition of the expanded perlite into smaller relatively high density perlite fragments.
The present invention provides an improved particle, suitable for mixing in commercial processes with polymeric materials that has a low density and sufficient strength to resist fracture and attrition during the mixing process.
The present invention provides an expanded particulate perlite coated with a cured silicon compound that is significantly superior to the properties of prior art materials.
Silicon coatings on expanded perlite particles are not unique to the present invention. In prior art techniques, silicon compounds have been applied to expanded perlite in an attempt to (1) increase the dispersion of filler in the resin, and (2) to lower the absorbency of resin into the porous structure of expanded perlite particles. None of the prior art techniques treat the silicon coating in the manner taught herein and therefore none of the prior art techniques yield the improved product that results from the practice of the present invention.